Multilateration System and Method

ABSTRACT

In the described system the multilateration process to be applied to determine an aircraft&#39;s position is chosen on the basis of the available returns, receiver geometry and the height of the aircraft.

This invention relates to a multilateration system for aircraftlocation.

Many types of aircraft transmit coded signals for example SecondarySurveillance RADAR (SSR) codes such as a so-called mode A, C or S codeswhich may be used by ground based receivers to determine the aircraft'sposition. The position is determined from noting the time of arrival atthe receivers and by using this with knowledge of the positions of thereceivers themselves. GB2250154A and GB 2349531A disclose suchmultilateration systems. These systems utilise four receiver stationscontrolled from one master station in order to establish the aircraft'sposition in three dimensions.

The present invention arose from a consideration of situations when onereceiver station fails to receive the transmitted code, the code isgarbled or when one receiver station develops a fault. Consideration wasalso given to situation where aircraft are unable to transmit mode Acodes.

According to the invention there is provided a multilateration systemcomprising a plurality of receiver stations for receiving signals fromaircraft and a controller to apply a multilateration process to outputsof the receiver stations, indicating receipt of the signal, to derive aposition of the aircraft characterised in that the controller determinesthe number of active receiver stations receiving the code, determinesthe type of code and performs a multilateration process in accordancewith the determination to provide a position of the aircraft.

In certain situations it will be appreciated that there may beinsufficient receiver stations receiving the transmitted code todetermine the location with a great deal of accuracy. For example, threereceiver stations will be able to provide a two dimensional positionwhich may in some circumstances be useful.

Some aircraft are equipped with mode A SSR transponder but are able totransmit other codes for example mode C. Mode A codes include a uniqueaircraft identifier and thus can distinguish a mode A code transmittedby one aircraft from second mode A code transmitted by another. In somecases the mode A transmission may be corrupted and hence not usable.Other codes may not include such a unique identifier. Preferably, insuch a case the multilateration process will include a reference totracking system to distinguish between possible sources. In the trackingsystem, a table is produced on the basis of the returned signals whichis revised over time.

It will be appreciated that a multilateration process involvessignificant computational resources and it will be advantageous in someapplications to perform the different multilateration processesavailable according to the accuracy required. Preferably, this isselected on the basis of the source aircraft's height. This has beenfound to be advantageous since the uncertainty in position in terms ofground position of the aircraft will increase with an increase inheight. Hence, when the aircraft is at a high altitude full threedimensional multilateration will be required whereas at a relatively lowaltitude two dimensional multilateration will suffice. In the describedembodiment, for heights between high and the low altitude thresholds atwo dimensional multilateration is performed which is augmented with theheight of the aircraft.

The transmitted code may include data concerning the height of theaircraft. This may be determined by the aircraft itself or by groundbased means. In the case of Secondary Surveillance Radar (SSR) codes, amode C code includes height information. In this case, the controllermay perform a two dimensional multilateration process using some of thereceivers and using the value of the height to arrive at a threedimensional location.

The invention also provides a multilateration method.

A specific embodiment of the invention will now be described by way ofexample only with reference to the drawing in which:

FIG. 1 shows in schematic form a multilateration system 1 operating inaccordance with the invention; and

FIGS. 2 to 7 are explanatory drawings.

As is shown in FIG. 1, a multilateration system 1 includes a pluralityof receiver stations 2 to 6 positioned at a number of locations on theground. These receive a transmitted signal include a code from atransponder mounted on an aircraft 7. The code is a SecondarySurveillance RADAR code which may be a mode A, mode S, mode C or may bean unknown mode of code.

Receiver station 4 is termed a master station because it includes acontroller which uses the data from the receivers to perform themultilateration process. (In alternative embodiments it need not beco-located with the receiver.) The data from the receiver stations 2, 3,5 and 6 is passed to the master station 4 over data links 8. Thecontroller 20 is microprocessor based and is shown in greater detail inFIG. 2. It includes a number of input ports 21 to 24 linked to the datalinks 8 and hence to the receiver stations. The input ports areconnected to a correlator 25 which forms the data into sets whichoriginate for particular transmissions of codes or events. The eventsare correlated by reference to time. Thus, if a mode A, mode C, mode Sor an unknown mode code arrive within a certain time frame then they areconsidered to originate from the same aircraft. The correlator also tiesup Time Of Arrival information from all receivers for a giventransmission as a so-called TOA Vector which may be of arbitrary lengthdependent upon the receivers that received a particular emission of acode. The vectors are then stored in memory 26 as tables of times ofarrival and associated codes.

The vector held in the memory 26 is then accessed by a locator 27. Thisincludes decision logic 28 which analyses the data to determine for eachvector a number of criterion, as will be described later, and then toselect the appropriate multilateration process to be applied to thedata. The vector together with an instruction as to the process to beapplied is then passed to a multilateration processor 28 and thepertinent multilateration process applied. The position is then used topopulate an entry in a track table 29. This is shown in FIG. 3. Each rowof the table is a termed a “track” and includes the codes whether modeA, C or S Airframe Address, and a position expressed as co-ordinates x,y and z. The track table is accessible to a plot association block 30.This is able to associate different responses from the aircraft to forma single track entry in the track table 30 from multiple track entries.

The plot association block 30 provides an output to a formatter 31 whichplaces the tracks into the correct format for input into a trackersystem 32. The tracker system 32 provides an output to an air trafficcontrol system 33 for displaying the tracks to a human air trafficcontrol officer.

The selection criterion referred to above include the following:

-   1. Mode type, whether the received signal is mode A, S or C or an    unknown mode.-   2. Number of receiver stations providing data to the data set.-   3. Whether the data indicates height or height is available from    another system or height may be assumed.-   4. Desired accuracy for the positioning

The height may be determined in a number of ways. If the received codeis mode C then this includes a height value provided by the aircraftitself by use of an onboard altimeter for example. (In some embodiments,height may be provided from an earlier multilateration on the sameaircraft or from knowledge of the aircrafts flight path which mayrequire the use of a particular altitude for example.)

As is shown in explanatory FIG. 4, the aircraft's height has anuncertainty and this will translate to an uncertainty in thecorresponding ground position. Arising from an appreciation of this, theinventors have determined that a satisfactory multilateration at lowlevels may be achieved by assuming that the aircraft is at zero altitudeand co-planar with the receivers and hence a two dimensionalmultilateration process may suffice. For intermediate levels, a twodimensional multilateration augmented with height information may beused and a three dimensional multilateration using four or morereceivers will be required at high altitudes. This banding is shownschematically in FIG. 5.

The decision logic 28 considers each vector in the manner illustrated inFIG. 6. The TOA vector is accessed and, in decision step 60, the vectorlength is considered. If the vector length is four returns or more thenbranch 61 is followed to the next step 62.

In step 62, the vector is considered and the receivers making thereturns determined. It will be appreciated that even though four returnsare available in a practical system these may not be ideally spread.Hence, if the geometry of the group of receivers providing the returnsin the vector are not such as to give sufficient accuracy then anegative branch 63 is followed to step 64. If the geometry does offersufficient accuracy then branch 65 is followed to step 66. In step 66 afull three dimensional multilateration is instructed.

Returning to step 60, if the vector length is less than four thennegative branch 67 is followed to step 64. In step 64, a decision ismade as to whether or not a two dimensional multilateration or anaugmented two dimensional multilateration is to be performed using thebarometric height indicated in a mode C emission. The step is dividedinto various choices dependent upon both the SSR code associated withthe data input and the certainty of which type the code might be. Thefollowing cases exist:

-   -   1. Mode S    -   2. Mode A    -   3. Mode C    -   4. Either Mode A or C (i.e. not known which)

Note that military variants have been ignored for clarity.

Each of these cases has its own decision logic. The logic used isdependent upon the extent of information in the Track Table. The processis illustrated for three cases as shown in FIG. 7 a to d.

In FIG. 7 a, the first case process is represented for an examination ofthe received code in which the code is a mode S code. In a firstconsideration step 70, the received code has an airframe address whichis compared with the tracks in the track table 29 to see if the airframeaddress is already present as an entry. If it is, then the processfollows branch 71 to the next consideration step 72 in which the tracktable is examined for the availability of the mode C altitude for theparticular track. If the mode C altitude is present, then the branch 73is followed and the next consideration step 74 made. In step 74,consideration is made as to whether or not the altitude is contained inthe current mode S received code. If it is, then branch 75 is followedand the track is used in process 76 to perform a two dimensionalmultilateration using the altitude in the mode S code and the track inthe track table is updated with the position in terms of x, y and zco-ordinates.

If in the step 74, the altitude is not contained in the current mode Sthen the negative branch 77 is followed to process 78. In this process amultilateration is performed with the altitude from the track tableavailable from the last mode C return. If the result is then passed to amatching process 79 which compares the result with one extrapolated forthe track. If there is a match within a certain threshold, then thetrack in the track table is updated with the new position. If there isno match, then the negative branch 80 is followed to process 81. (Inessence this will be because the received code is a new aircraftentering the air traffic control area.) The step 81 results in amultilateration process being performed at zero feet and the track tableis updated to include a new track bearing a flag indicating that it isnot to be output from the system as it has insufficient positionalaccuracy. This track will be updated as more codes are received and whenthe accuracy is acceptable the flag will be brought down permitting thetrack to be output from the system.

Returning back to step 70, if the airframe address is not present in thetrack table then negative branch 82 is followed to the step 74. If thealtitude is contained in the current mode S code, then the positivebranch 83 is followed to step 76. If the attitude is not contained thena negative branch 84 is followed to process 81.

Returning to step 72 if the result of the consideration of the mode Caltitude being in the track table is negative then branch 85 is followedto step 74.

In step 78, the multilateration process is performed using the altitudein the track table. However in step 76 the multilateration process willbe carried out on the basis of that in the current mode S code. Themultilateration process to be applied whether 2d or 2d augmented is donewith a consideration of the bandings of FIG. 5. If the height is betweenHmin and H3d then a 2d assisted multilateration process is followed anda flag is added indicating reduced accuracy. If the height is below hminthen the multilateration process is a 2d process and the result ismarked by a flag as full accuracy. If the height is above h3d then thelocator does not provide an output and the track table is not updated.

In case 2 shown in FIG. 7 b the currently received code is a mode Acode. This code is compared in step 90 with each track in the tracktable. The matches are then considered in turn in step 91 as to whetheror not the track includes a Mode C altitude. If it does then branch 92to process 93 is followed or if not branch 94 to process 95. If thereare no matches then branch 96 is followed to process 97.

In process 93, the height is compared with the banding as before toselect the multilateration process to be applied. If the height isbetween hmin and h3d then a 2d assisted multilateration process isapplied using the altitude from the track table for the matching trackflagging the result as reduced accuracy. If the height from the tracktable is below hmin then a 2d multilateration is performed marking theresults as full accuracy. if the height is above h3d then there is nooutput from the locator.

In process 95 a 2d multilateration is carried out and the result flaggedas not to be output from the system. The results for position fromprocess 93 and 95 are passed to a comparison step 96. In this comparisonstep the position in terms of x, y and z co-ordinates is compared withan extrapolated position for the track. If there is a match the resultsare used to update the track in the track table if there is no matchthen the loop branch to the step 90 is followed. (Matching may be donein terms of x, y and z or x, y and a z determined from a mode Ctransmissions in some embodiments.)

In the case of no matches in the code or on the x, y, z co-ordinates inprocess 96, a 2d multilateration process is carried out and the resultsmarked as not to be output from the system.

In the case of the received code being a mode C code the steps are shownin FIG. 7 c. In a first step 100, the matches for the received code inthe track table are identified. Then each match has a multilaterationprocess 101 applied to it using the altitude from the track entry in thetrack table. If the altitude is between hmin and h3d then a 2D assistedmultilateration is carried out with the results mark as reducedaccuracy. If the altitude is below hmin then a 2d multilaterationprocess is carried out and marked as full accuracy. If the altitude isover h3d then no output results from the locator. The resultantmultilateration position is compared with process 102 to an extrapolatedposition for the particular track form the track table. If there is amatch then the track is updated. If there is not a match, then thenegative branch is followed back to the step 100.

If there are no matches for the code or co-ordinates then process step104 is carried out for the received mode C code as it is being receivedfrom an aircraft entering the monitored airspace. In process 104 a 2 dor 2d assisted multilateration process is carried out on the basis ofthe altitude in the received mode C code. In this process if thealtitude is between hmin and h3d a 2d assisted multilateration processis carried out with the result marked as reduced accuracy. If thealtitude is below hmin then a 2d multilateration process is carried outand the results marked as full accuracy. If the height is above h3d thenthere is no output from the locator.

The 3D multilateration process will involve four or more of thereceivers as disclosed in GB225014 or GB 239531 for example. However, ifthe height is known, only two time difference of arrival figures need bedetermined.

1. A multilateration system comprising a plurality of receiver stationsfor receiving signals from aircraft and a controller to apply amultilateration process to outputs of the receiver stations, whichoutputs indicating receipt of the signal, to derive a position of theaircraft characterized in that the controller determines the number ofactive receiver stations receiving the code, determines the type of codeand performs a multilateration process in accordance with thedetermination to provide a position of the aircraft.
 2. A system asclaimed in claim 1 wherein the controller performs a multilaterationprocess using outputs of the receivers and the height of the aircraft.3. A system as claimed in claim 1 wherein the multilateration processapplied is chosen with reference to a determined height for theaircraft.
 4. A system as claimed in claim 1 wherein the outputs are usedto create a track for the aircraft held in memory.
 5. A system asclaimed in claim 4 wherein the track comprises at least one of a modetype indicator, a mode code, a position.
 6. A system as claimed in claim1 comprising control logic for determining the number of receiversproviding usable returns, determining from the returns the geometry ofthe receivers making the returns and whether the geometry will support athree dimensional multilateration and instructing such a multilaterationto be carried out, and in the absence of such a determinationinstructing a two dimensional or two dimensional augmentedmultilateration to be carried out.
 7. A system as claimed in claim 4,comprising logic for comparing a currently determined position providedby the multilateration process carried out with a predicted positionextrapolated from a track and in the event of the comparison beingwithin a certain threshold concluding that there is a match and updatingthe track with the currently determined position.
 8. An air-trafficcontrol system for monitoring air-traffic comprising a multilaterationsystem as claimed in claim 1 and means to display at least some of thetrack information provided by the multilateration system to an operator.9. A multilateration method of determining the position of an aircraftemitting a code which method comprising: receiving the code at aplurality of receivers; determining time of arrival data for arrival ofthe code at the receivers; and applying one of a three, a twodimensional or a two dimensional, assisted by height data,multilateration process.
 10. A method as claimed in claim 9 wherein thechoice of multilateration process is also dependent upon a value for theheight of the aircraft.
 11. A method as claimed in claim 10 wherein theheight is a barometric height obtained from a received code.
 12. Amethod as claimed in claim 1 wherein the height is compared withthreshold values to determine the multilateration process to be used.13. A method as claimed in claim 1 wherein the data is stored as atrack.
 14. A method as claimed in claim 13 wherein a current positionprovided by the used multilateration process is compared withextrapolated positions from the tracks to determine a match and in theevent of a match updating the track with the current position.
 15. Anair-traffic control method comprising determining aircraft positionsusing a method as claimed in claim 1 and displaying the aircraftpositions to an operator.
 16. (canceled)